The Organism: Listeria monocytogenes is an uncommon cause of illness in the general population. However, in some groups, including neonates, pregnant women, elderly persons, immunosuppressed transplant recipients, and others with impaired cell-mediated immunity, it is an important cause of life-threatening bacteremia and meningoencephalitis. Growing interest in this organism has resulted from foodborne outbreaks and concerns about food safety.

Laboratory diagnosis: L. monocytogenes is a small, facultatively anaerobic, nonsporulating, catalase-positive, oxidase-negative, gram-positive bacillus that grows readily on blood agar, producing incomplete beta-hemolysis. The bacterium possesses one to five polar flagellae and exhibits a characteristic tumbling motility at 25°C. Optimal growth occurs at 30 to 37°C, but L. monocytogenes grows better than other bacteria at refrigerator temperatures (4 to 10°C), and by so-called cold enrichment can be separated from other contaminating bacteria by long incubation in this temperature range. Selective media have been developed to isolate the organism from specimens containing multiple species (food, stool) and appear superior to cold enrichment. When grown on blood-free agar and viewed with light transmitted at a 45-degree angle (Henry's illumination), colonies of L. monocytogenes appear blue, whereas other bacterial colonies appear yellowish or orange. Routine media are effective for isolating L. monocytogenes from specimens obtained from normally sterile sites (cerebrospinal fluid [CSF], blood, joint fluid), but media typically used to isolate diarrhea-causing bacteria from stool cultures inhibit listerial growth. Listeria monocytogenes grows best at a neutral to slightly alkaline pH and dies at a pH below 5.5.

Clinical presentation: Except for vertical transmission from mother to fetus and rare instances of cross-contamination in the delivery suite or newborn nursery, human-to-human infection has not been documented. Infection most likely begins after ingestion of the organism in a foodborne source. The oral inoculum required to produce clinical infection is unknown; experiments in healthy mammals indicate that 10⁹ organisms or more are required.

Pregnant women are prone to develop listerial bacteremia, and twenty-two percent of perinatal infections result in stillbirth or neonatal death. However, early diagnosis and antimicrobial treatment can result in the birth of a healthy infant. When in utero infection of the fetus occurs, it may also precipitate spontaneous abortion and the fetus may be stillborn or die within hours of a disseminated form of listerial infection known as granulomatosis infantiseptica characterized by widespread microabscesses and granulomas, particularly prevalent in the liver and spleen. In this entity, abundant bacteria are often visible on Gram stain of meconium.

In a compromised host, bacteremia without an evident focus has been the most common manifestation of listeriosis. Meningitis is second in frequency. Clinical manifestations are similar to those seen in bacteremia with other causes and typically include fever and myalgias. A prodromal illness with diarrhea and nausea may occur. Transient bacteremias in healthy persons may go undetected. L. monoytogenes has tropism for the brain itself, particularly the brain stem, as well as for the meninges. Many patients with meningitis have altered consciousness, seizures, or movement disorders, or all of these, and truly have a meningoencephalitis. It is the fifth most common cause of meningitis behind H. influenzae, S. pneumoniae, N. meningitidis, and group B streptococcus but has the highest mortality at 22%. An unusual form of listerial encephalitis involves the brain stem, and in contrast to other listerial CNS infections, this illness usually occurs in healthy adults. The typical clinical picture is one of a biphasic illness with a prodrome of fever, headache, nausea, and vomiting lasting about 4 days followed by the abrupt onset of asymmetric cranial nerve deficits, cerebellar signs, and hemiparesis or hemisensory deficits, or both. Mortality is high, and serious sequelae are common in survivors.

Other symptoms and findings associated with listerial infections include macroscopic brain abscesses, endocarditis, focal infections resulting in conjunctivitis, skin infection, and lymphadenitis. Bacteremia can lead to hepatitis and hepatic abscess, cholecystitis, peritonitis, splenic abscess, pleuropulmonary infection, joint infection, osteomyelitis, pericarditis, myocarditis, arthritis, and endophthalmitis.

Treatment: In the absence of a positive CSF Gram stain, initial therapy for bacterial meningitis in all adults older than 50 years should include either ampicillin or trimethoprim-sulfamethoxazole, especially if there is no associated pneumonia, otitis, sinusitis, or endocarditis that would point to causes other than L. monocytogenes. Clinically significant antimicrobial resistance has not been encountered, but vigilance is warranted since transfer of resistance from enterococci to L. monocytogenes has been documented.
 

REFERENCES
Gellin BG, Broome CV. Listeriosis. JAMA. 1989;261:1313-1320.

Skoberg K, Syrjanen J, Jahkola M, et al. Clinical presentation and outcome of listeriosis in patients with and without immunosuppressive therapy. Clin Infect Dis. 1992;14:815-821.

Koneman et al. Color Atlas and Textbook of Diagnostic Microbiology, 5^th Edition, Lippincott-Raven, Philadelphia, 1997.
 

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